1: make a skybox which features mountains2: use a bigger terrain mesh and make mountains on the edges3: make a small model of mountains with a hole in the middle and no collisions then scale it really big in the editor so it surrounds your terrain and looks like mountains in the distance (how CoD did it).

Duion wrote:Skyboxes are not really needed anymore, they were mostly relevant in old times where computers were slow, now you can make a real big mesh terrain as background without that much cost.

But still skyboxes are more easy to set up than some huge terrains. It is also much faster. Also it makes your game more play-able on computers with lower specs. I for one prefer Skyboxes over big terrain. In that way I can control the sucking memory demon from draining all the energy from a small laptop. You see that is awesome. Skyboxes are therefore great tools to create a believable illusion of something great in the horizon with.

Skyboxes has also been used in games such as Dragon Age: Origin. For the trained eye such things are easy to spot